Aluminum family

Photo by: Andrey Zyk

The aluminum family consists of elements in Group 13 of the periodic
table: boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium
(Tl). The family is usually named after the second element, aluminum,
rather than the first, boron, because boron is less typical of the family
members than is aluminum. Boron is a metalloid (an element that has some
of the properties of metals and some of the properties of nonmetals),
while the other four members of the family are all metals.

Aluminum

Aluminum is a lightweight, silvery metal, familiar to every household in
the form of pots and pans, beverage cans, and aluminum foil. It is
attractive, nontoxic, corrosion-resistant, nonmagnetic, and easy to form,
cast, or machine into a variety of shapes. It has a melting point of
660°C (1,220°F) and a boiling point of 2,519°C
(4,566°F).

Aluminum is the third most abundant element in Earth's crust after
oxygen and silicon, and it is the most abundant of all metals. It
constitutes 8.1 percent of the crust by weight and 6.3 percent of all the
atoms in the crust. Because it is a very active metal, aluminum is never
found in its metallic form. Rather, it occurs in a wide variety of earthy
and rocky minerals, including feldspar, mica, granite, and clay. Kaolin is
an especially fine, white, aluminum-containing clay that is used in making
porcelain.

Known as aluminium in other English-speaking countries, the element was
named after the mineral alum, one of its salts that has been known for
thousands of years. Alum was used by the Egyptians, Greeks, and Romans as
a mordant, a chemical that helps dyes stick to cloth.

Properties and uses.
Pure aluminum is relatively soft and not the strongest of metals. When
melted together with other elements such as copper, manganese, silicon,
magnesium, and zinc, however, it forms alloys (a substance composed of two
or more metals or of a metal and a nonmetal) with a wide range of useful
properties. Aluminum alloys are used in airplanes, highway signs, bridges,
storage tanks, and buildings. The world's tallest buildings, the
World Trade Center towers in New York, are covered with aluminum. Aluminum
is being used more and more in automobiles because it is only one-third as
heavy as steel and therefore decreases fuel consumption.

In spite of the fact that aluminum is chemically very active, it does not
corrode in moist air the way iron does. Instead, it quickly forms a thin,
hard coating of aluminum oxide. Unlike iron oxide or rust, which flakes
off, the aluminum oxide sticks tightly to the metal and protects it from
further oxidation. The oxide coating is so thin that it is transparent, so
the aluminum retains its silvery metallic appearance. Sea water, however,
will corrode aluminum unless it has been given an unusually thick coating
of oxide by the anodizing process. (During the anodizing process, a piece
of aluminum is oxidized in order to create on its surface a coating of
aluminum oxide, which is able to take dyes, unlike plain aluminum.)

When aluminum is heated to high temperatures in a vacuum, it evaporates
and condenses onto any nearby cool surface such as glass or plastic. When
evaporated onto glass, it makes a very good mirror. Aluminum has largely
replaced silver in the production of mirrors because it does not tarnish
and turn black as silver does when exposed to impure air. Many
food-packaging materials and shiny plastic novelties are made of paper or
plastic with an evaporated coating of bright aluminum. The silver-colored
helium balloons popular at birthday parties are made of a tough plastic
called Mylar™, covered with a thin, evaporated coating of aluminum
metal.

Aluminum is one of the best conductors of electricity, with a conductivity
about 60 percent that of copper. Because it is also light in weight and
highly ductile (able to be drawn out into thin wires), it is used instead
of copper in almost all of the high-voltage electric transmission lines in
the United States.

Aluminum is used to make kitchen pots and pans because of its high heat
conductivity. It is handy as an airtight and watertight food wrapping
because it is very malleable; it can be pressed between steel rollers to
make foil (a thin sheet) less than one-thousandth of an inch thick. Claims
are occasionally made that aluminum is toxic and that aluminum cookware is
therefore dangerous, but no clear evidence for this belief has ever been
found. Many widely used over-the-counter antacids contain thousands of
times more aluminum (in the form of aluminum hydroxide) than a person
could ever get from eating food cooked in an aluminum pot. Aluminum is the
only light element that has no known physiological function in the human
body.

Production.
As a highly reactive metal, aluminum is very difficult to separate from
other elements that are combined with it in its minerals and compounds. In
spite of its great abundance on Earth, the metal itself remained unknown
for centuries. In 1825, some impure aluminum metal was finally isolated by
Danish physicist Hans Christian Oersted (1777–1851) by treating
aluminum chloride with potassium amalgam (potassium dissolved in mercury).
Then, in 1827, German chemist Hans Wöhler (1800–1882)
obtained pure aluminum by the reaction of metallic potassium with aluminum
chloride. He is generally given credit for the discovery of elemental
aluminum.

But it was still very expensive to produce aluminum metal in any quantity,
and for a long time it remained a rare and valuable metal. In 1852,
aluminum was selling for about $545 a pound. The big breakthrough came in
1886, when Charles M. Hall, a 23-year-old student at Oberlin College in
Ohio, and Paul L-T. Héroult, another college student in France,
independently invented what is now known as the Hall or
Hall-Héroult process. This process consists of dissolving alumina
(aluminum oxide) in melted cryolite, a common aluminum-containing mineral,
and then passing an electric current through the hot liquid. Molten
aluminum metal collects at the cathode (negative electrode). Not long
after the development of this process, the price of aluminum metal
plummeted to about 30 cents a pound. The process used to extract aluminum
from its ores today is essentially the same as that developed by Hall and
Héroult 150 years ago.

Boron

Elemental boron occurs in a variety of forms, ranging from clear red
crystals to a black or brown powder to a transparent black crystal that is
nearly as hard as diamond. The element is never found free in nature but
is extracted commercially from minerals such as borax, ulexite,
colemanite, and kernite. Boron is a relatively rare element, constituting
about 0.001 percent of Earth's crust. It ranks number 38 in
abundance, after nitrogen, lithium, and lead, but before bromine, uranium,
and tin.

Properties and uses.
The physical properties of boron are somewhat difficult to determine
since the element occurs in so many different forms. The melting point of
its most stable form is given as 2,180°C (3,900°F) (the
second highest after carbon); its boiling point is about 3,650°C
(6,600°F).

Chemically, boron is a fascinating element. One text on the chemical
elements claims that the inorganic chemistry of boron is "more
diverse and complex than that of any other element in the periodic
table." The element forms five types of compounds: (1) metal
borides (a metal plus boron), (2) boron hydrides (boron plus hydrogen),
(3) boron trihalides (boron plus a halide; a halide is a simple halogen
compound), (4) oxo compounds (boron plus complex oxygen radicals; a
radical is a group of atoms that behaves as a unit in chemical reactions
but is not stable except as part of the compound), and organoboron
compounds (boron combined with an organic, or carbon-containing,
component).

Boron itself has relatively few uses aside from its role in nuclear
reactors as a neutron absorber and in alloys as a hardening agent.
(Nuclear reactors are devices used to control the energy released from
nuclear reactions.) It is also used in the manufacture of semiconductors.
(Semiconductors are substances that conduct an electric current but do so
very

Aluminum alloy framing on a building.
(Reproduced by permission of

JLM Visuals

.)

poorly.) Probably its best known compound, borax, is used as a water
softening agent, in the production of glasses and ceramics, and as an
herbicide. A compound derived from borax—boric acid—is used
as an eyewash and in the production of heat-resistant glass.

Two boron compounds of special interest are boron carbide and boron
nitride. Both are used as refractories, substances that are highly
resistant to heat. The melting point of boron carbide is about
2,350°C (4,230°F) and that of boron nitride, over
3,000°C (5,400°F). When boron nitride powder is compressed
at very high pressures, it produces a hard crystalline material that is as
hard as natural diamonds.

Gallium, indium, and thallium

For most of its history, gallium was best known for one unusual physical
property: it has a melting point of 29.76°C (85.6°F), less
than that of the human body. If you were to hold a lump of gallium metal
in your hand, therefore, it would melt.

In spite of this fact, gallium and its compounds have traditionally had
few uses—until recently. In the 1970s, a compound of gallium called
gallium arsenide was found to have semiconductor properties. Gallium
arsenide has also been used extensively in light-emitting diodes (LEDs),
which are used in the electronic displays of calculators, watches, and CD
players.

Neither indium nor thallium has many commercial applications. The former
element is used largely in making alloys and in the production of
transistors and photo cells. A radioactive isotope of the latter,
thallium-201, is used in medical diagnostic studies, especially those
involving the function of the circulatory system.